JAMES PRESCOTT JOULE 159
192 revolutions with one cell, and 283 and 321 with two
cells. Hence a laminated armature generally gave a large
increase in revolutions with the correct intensity of current.
In his third paper he describes the results obtained with
armatures made of rectangular wires in order to avoid the
wastage of space in the packing of circular wires. The
rectangular wire armature gave 162 revolutions, whereas
the corresponding solid one gave 130. This is followed by
more investigations of the construction and properties of
magnets. £'The resistance which iron opposes to the
instantaneous induction of magnetism is of considerable
importance." He finds that a steel magnet could attract
a small magnet attached to the arm of a balance with a force
of 32 grains at a distance of half an inch, and could lift a
weight of sixty ounces. One of his electro-magnets exerted
an attraction of only 5-1 grains at half an inch, but could
lift ninety-two ounces. On the basis of his measurements
he gives various rules for the efficient design of electro-
magnets.
In his fourth paper, dated May 28th, 1839, he describes
a galvanometer which he has calibrated by electrolysis. The
current which deflected the needle was sent through a water
voltameter and the size of the deflections compared with
the volume of mixed gases evolved. With this apparatus
he measured the attractive power of numerous electro-
magnets excited by currents of various strengths and tabu-
lated his results. From the figures he deduced a new law,
that "The attractive force of two electro-magnets for one another
is directly proportional to the square of the electric force to which
the iron is exposed; or if E denote the electric current, W the
length of wire and M the magnetic attraction^ M=E2fF'2."
At the end of the paper he writes that "I can hardly
doubt that electro-magnetism will ultimately be substituted
for steam to propel machinery. If the power of the engine
is in proportion to the attractive force of its magnets, and
if this attraction is as the square of the electric force, the
economy will be in the direct ratio of the quantity of elec-
tricity, and the cost of working the engine may be reduced
ad infinitum. It is, however, yet to be determined how far